The invention relates to an (individual) sensor for electrically measuring a force acting on the sensor which is distributed unevenly over a measurement surface of the sensor.
Personal scales are suitable for measuring a force acting on them, e.g. a force corresponding to the weight of a user, even if this force is distributed unevenly over the rigid standing surface. Such measurement becomes problematic when it is desired to measure a force applied by a user, e.g. in a shoe. Such measurement is necessary in the case of various medical indications, in particular when the person “monitored” is supposed to perform a specific exercise and has to access the force applied easily.
A capacitive measuring arrangement for determining forces or pressures in which a large number of individual sensors are distributed over a surface, e.g. an insole in a shoe, is known from DE 36 34 855 C1. The sensors in the case of the known arrangement consist of capacitors, i.e. of a large number of capacitor surfaces which are constructed in a matrix and are scanned individually as a matrix. A total force or a total pressure can be calculated from the sum of the individual forces or individual pressures. In this case, it hardly matters whether individual regions of the entire surface are under a greater or lesser load since the total surface is divided into smaller partial surfaces, so that partial forces can be measured and added up. If in the case of this arrangement the acting forces are distributed beyond the measurement surface, i.e. if the total sensor surface is too small, a correct measurement result cannot be derived.
When an elastic sensor (as mentioned above) is required, the measurement results in the case of full-surface sensor elements, e.g. capacitor films, are extremely inaccurate if the force is distributed unevenly.
It is an object of the invention to set forth a sensor of the type referred to first hereinbefore such that a correct total force measurement can be derived in a simple manner.
This object is achieved by a sensor according to claim 1. Particular uses of the sensor are named in claim 12.
In particular, this object is achieved by a sensor for electrically measuring a force (F) acting on the sensor within a specified measuring range, which force is distributed unevenly over a measurement surface (10) of the sensor, wherein an electrical force measurement signal is generated, comprising
an areally formed spring device (20) which is arranged between the measurement surface (10) and a counter-surface (11),
wherein a first measuring element (12) is arranged in or on the measurement surface (10) and a second measuring element (13) is arranged in or on the counter-surface, the elements substantially completely covering these surfaces in each case,
wherein the measurement surface (10) and the counter-surface (11) plus the measuring elements (12, 13) are designed to be elastically deformable,
wherein the measuring elements (12, 13), e.g. capacitor plates, are designed such that the measuring signal can be generated from a distance between the measuring elements (12, 13),
wherein the spring device (20) has a large number of incompressible but elastically designed spring members (21 to 21n) which are arranged spaced apart from each other by gaps (22) in such a way that each spring member (21 to 21n) upon loading by the force (F) or a fraction of this force (F) can deform, into the gaps (22) and hence in a space-consuming manner, and
wherein each spring member is designed such that its height between the measurement surface and the counter-surface upon loading of the sensor by the force (F) within the specified measuring range is linearly proportional to a partial force acting thereon.
Surprisingly, it has turned out that when the sensor is loaded by forces within a specified measuring range it is not necessary to operate with linearisation “over everything” as with rigid sensors, with which an even distribution of forces onto all the regions of the sensor takes place, regardless of how the effective forces are distributed. Depending on the specified measuring range, then the overall arrangement can be such that the spring members have a linear characteristic within measuring ranges of small area as well. As a result, it is possible to measure a force sum by means of the sensor, namely substantially independently of the distribution of the effective partial forces.
Furthermore, the measurement surface and the counter-surface plus the measuring elements are designed to be deformable, namely in particular elastically deformable. Therefore it is not—as in the case for example of personal scales—rigid surfaces which are used as the measurement surface and counter-surface, but elastically resilient surfaces, so that for example an insole in a shoe can be designed as a sensor. Preferably in this case the measurement surface and/or the counter-surface are designed as a textile material or comprise a textile material, which may be in particular a knitted fabric or a woven fabric. Such materials are known per se. Particularly preferred is an embodiment in which the measuring elements comprise capacitor plates, with a capacitance between the capacitor plates being able to be measured to generate the force measurement signal. The technology necessary for this is already very well developed.
At this point it should be pointed out that the measuring elements may also be constructed as inductors or as ohmic resistors (or mixtures thereof), and that capacitor plates or capacitors as measuring elements are discussed as a preferred example of embodiment in the following description.
The measurement surface and/or the counter-surface are connected to the spring members by fastening means. These fastening means may be designed in diverse ways. What is important in this case is however that the measurement surface and the counter-surface be connected to the spring members such that the geometric overall arrangement remains substantially constant. In this case, the fastening means are preferably designed elastically such that the measurement surface and/or the counter-surface is/are elastically displaceable relative to the spring devices. This is necessary in particular when curving of the measurement surface or of the counter-surface occurs. Fastening means which are displaceable in such a manner may for example be elastic bonded joints.
The measurement surface and the counter-surface preferably have in addition to the first or second measuring element respectively, on their sides remote from the spring device, shielding elements for electrically shielding the measuring elements. As a result, even relatively small measuring signals can be detected with little disturbance.
The spring members may be constructed as geometric bodies in various ways. They may comprise bar-shaped, frusto-pyramidal or frusto-conical individual elements, “frusto-pyramidal” or “frusto-conical” also being taken to mean cuboids or cylinders with very slight angling. These individual elements are arranged at regular distances from each other. In particular silicone rubber is a suitable material here, with Shore hardnesses of 45-55, in particular 47-53, being preferred. Adaptation to the forces to be measured may of course be carried out here.
The spring members may—as described above—be connected together by a bonded joint, or alternatively by a carrier surface from which the individual spring members protrude either on one side or on both sides (with the carrier surface in the middle). In both cases, production of the sensor is particularly simple and precise when the carrier surface is designed in one piece with the spring members.
If the measuring elements comprise capacitor plates, preferably a capacitance measuring device is provided which is connected to the shielding elements and the capacitor plates and is designed such that capacitances between the capacitor plates and the shielding elements can be measured. Since—in a manner known per se—all the capacitor plates and also the shielding means are separated from one another by dielectrics, a measuring signal can be generated herefrom which corresponds substantially to the temperature-dependent material properties of the dielectrics, i.e. in particular the material-dependent and temperature-dependent dielectric constant. By means of this measuring signal, the force measurement signal can be corrected with respect to its temperature dependency in a correction device.
Many different combinations of form and material which permit the aforementioned linearisation are conceivable for the spring elements. In one embodiment, the spring members are assembled from spring members of different geometric construction in groups of spring members which in each case have different spring characteristics from each other, such that they compensate for each other and bring about linearisation of the overall spring characteristic of the group.
A sensor of the type shown here can advantageously be used for a large number of measurement tasks. In particular, these are those measurement tasks in which the surfaces between which the forces occur are curved or—and this is even more difficult with regard to the sensors to be selected—variable during the course of the measurement. In particular, they are in this case measurements of loads exerted by human limbs or effectors of a robot on a surrounding object, e.g. a shoe, a prosthesis, an orthosis, a handle, a steering wheel or a natural or artificial joint, and also tools or workpieces. These can also be taken to mean forces exerted by body parts on subjacent supporting structures, e.g. a car seat, a mattress, a lounger or a riding saddle.